A bug has been found in the installer, which accidentally removes /etc/apt/sources.list.d/aptosid.list if adding non-free Firmwares gets selected. Not selecting this option avoids this bug and allows adapting /etc/apt/sources.list.d/debian.list afterwards, in order to install firmware or other non-free packages.

Since I would like to get nonfree installed during installation,
can I go ahead with nonfree selected and then add the aptosid.list after installation? Rather than what is suggested.

If you did not see add non-free during installation it is most likely because the installer did not detect any hardware in the system for which non-free modules would be needed. That's a good thing! It means that your system is supported natively by aptosid.
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Neal